Moment-tensor inversions for local earthquakes using surface waves recorded at TERRAscope

We have developed a method to determine moment tensors for local earthquakes using short period (10 to 50 sec) surface waves recorded at TERRAscope stations. To correct for the substantial lateral variations in crustal structure, we applied phase corrections to the data using a regionalized phase-velocity model. We have determined moment tensors for over 180 events in the last 3 yr in southern California for magnitudes as small as 3.2 and as large as 6.5. The results are consistent with those obtained from first-motion data as well as other waveform inversions. When continuous data telemetry from the stations becomes available this method can yield moment tensors for earthquakes in southern California and adjacent regions within minutes after the occurrence of an event. Our results confirm the relation log M_o (seismic moment) ∝ 1.5M_L (local magnitude) obtained by an earlier study

1. Using Short-Period Surface Waves to Study Seismic Source and Structure. 2. Source Complexity of Large Strike-Slip Earthquakes

The availability of high dynamic range very broad band seismic data in recent years has greatly increased the level of detail and the speed with which we can study the seismic source. The work presented in this thesis draws heavily on the deployment of broad-band seismometers, both on a worldwide scale, with networks like IRIS, IRIS/IDA and GEOSCOPE, as well as on a local scale, using data from the TERRAscope network. The routine study of seismicity in Southern California, like in other seismically active regions, has traditionally been carried out using dense arrays of high-gain short-period seismometers. With the addition of the very broad band instrumentation of TERRAscope we can improve this pursuit in several ways, one of which being the use of short period surface waves to study local earthquakes as described in chapter 1 of this thesis. Over the years, surface waves have proved to be very reliable and stable for moment tensor inversions. The method is very rapid, and because of the longer periods used they are more reliable for consistent estimation of earthquake moment. At short distances the surface waves arrive within a few minutes after an event has occurred at the stations, and with real-time telemetry we can obtain the size and mechanism for local earthquakes within minutes. The propagation corrections for surface waves are very straightforward so that this procedure can be made completely automatic. Armed with the results from above procedure, we can determine travel time residuals for a dense distribution of raypaths across Southern California. In chapter 2 we present tomographic inversions of these resid­uals, for Love and Rayleigh waves at periods between 10 and 50 seconds. The results indicate that lateral variations of phase velocity of up to 10% exist in the area, and that these anomalies can have relatively short wavelengths. The 1994 Northridge earthquake provided a wealth of data to apply our moment tensor inversion to, and in chapter 3 we present a detailed analysis of the aftershock mechanisms in relation to the source complexity of the mainshock. We show that the orientation of the aftershock mechanisms changes away from the zone where rupture took place. We suggest that this change in mechanism reflects changes in fault geometry which have limited the extent of the Northridge rupture, leading to a high static stress drop. The issue of source complexity is discussed further in chapter 4, where we present a systematic study of the rupture of three large strike-slip earthquakes and compare these results with observation on the surface rupture. We find a very good correlation which suggests that the source complexity can be attributed to fault geometry, which tends to become simpler as slip accumulates along a fault. This provides an explanation for the high stress drops that are observed for earthquakes which occur along faults with low strain rates. Finally, in chapter 5 we compiled energy and moment estimates for earthquakes in Southern California, based on the results in the previous chapters. We find that the radiated seismic energy is not linearly related to the seismic moment, but that instead the energy-moment ratio increases as a function of moment. We provide some suggestions as to the cause of this relationship, including a moment dependence of the specific fracture energy and a non-similarity of the frictional stress between different size earthquakes.</p

Seismic source and structure estimation in the western Mediterranean using a sparse broadband network

We present a study of regional earthquakes in the western Mediterranean geared toward the development of methodologies and path calibrations for source characterization using regional broadband stations. The results of this study are useful for the monitoring and discrimination of seismic events under a comprehensive test ban treaty, as well as the routine analysis of seismicity and seismic hazard using a sparse array of stations. The area consists of several contrasting geological provinces with distinct seismic properties, which complicates the modeling of seismic wave propagation. We started by analyzing surface wave group velocities throughout the region and developed a preliminary model for each of the major geological provinces. We found variations of crustal thickness ranging from 45 km under the Atlas and Betic mountains and 37 km under the Saharan shield, to 20 km for the oceanic crust of the western Mediterranean Sea, which is consistent with earlier works. Throughout most of the region, the upper mantle velocities are low which is typical for tectonically active regions. The most complex areas in terms of wave propagation are the Betic Cordillera in southern Spain and its north African counterparts, the Rif and Tell Atlas mountains, as well as the Alboran Sea, between Spain and Morocco. The complexity of the wave propagation in these regions is probably due to the sharp velocity contrasts between the oceanic and continental regions as well as the the existence of deep sedimentary basins that have a very strong influence on the surface wave dispersion. We used this preliminary regionalized velocity model to correct the surface wave source spectra for propagation effects which we then inverted for source mechanism. We found that this method, which is in use in many parts of the world, works very well, provided that data from several stations are available. In order to study the events in the region using very few broadband stations or even a single station, we developed a hybrid inversion method which combines P_(nl) waveforms synthesized with the traditional body wave methods, with surface waves that are computed using normal modes. This procedure facilitates the inclusion of laterally varying structure in the Green's functions for the surface waves and allows us to determine source mechanisms for many of the larger earthquakes (M > 4) throughout the region with just one station. We compared our results with those available from other methods and found that they agree quite well. The epicentral depths that we have obtained from regional waveforms are consistent with observed teleseismic depth phases, as far as they are available. We also show that the particular upper mantle structure under the region causes the various P_n and S_n phases to be impulsive, which makes them a useful tool for depth determination as well. Thus we conclude that with proper calibration of the seismic structure in the region and high-quality broadband data, it is now possible to characterize and study events in this region, both with respect to mechanism and depth, with a limited distribution of regional broadband stations

Initial investigation of the Landers, California, Earthquake of 28 June 1992 using TERRAscope

The 1992 Landers earthquake (M_s =7.5, M_w =7.3) was recorded at six TERRAscope stations in southern California. Peak accelerations ranged from 0.16 g at SVD (Δ=63 km) to 0.0092 g at ISA (Δ=245 km), decreasing with distance away from the fault zone. The peak velocity showed a different pattern reflecting the rupture directivity from south to north. The largest peak velocity, 19 cm/sec, was observed at GSC (Δ=125 km). Moment tensor inversion of long‐period surface waves yielded a mechanism with M_0=1.1×10^(27) dyne‐cm (M_w =7.3), dip=74°, rake=−176°, and strike=340°. Inversion of teleseismic P and S waves revealed two distinct sub‐events of 6 and 8 sec duration and about 10 sec apart. The source parameters for the first and second events are: M_0=1.9×10^(26) dyne‐cm, dip=83°, rake=179°, strike=359°; and M_0=6.1×10^(26) dyne‐cm, dip=87°, rake=178°, strike=333°, respectively. The radiated wave energy, E_S, was estimated as 4.3×10^(23) ergs. The ratio E_s/M_0=3.9×10^(−4) corresponds to a stress drop of 280 bars, and suggests that the Landers earthquake belongs to the group of high stress drop earthquakes, and occurred on a fault with a long recurrence time. The rupture directivity can be seen clearly in the records from PFO (Δ=68 km) located to the south and GSC located to the north of the epicenter. The maximum displacement at PFO is only 13% of that at GSC despite the shorter epicentral distance to PFO than to GSC. The slip distribution determined with the empirical Green's function method indicates that the Landers earthquake consists of two distinct sub‐events about 30 km apart, with the second sub‐event to the north being about twice as large as the first one. This slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

A global probabilistic tsunami hazard assessment from earthquake sources

Large tsunamis occur infrequently but have the capacity to cause enormous numbers of casualties, damage to the built environment and critical infrastructure, and economic losses. A sound understanding of tsunami hazard is required to underpin management of these risks, and while tsunami hazard assessments are typically conducted at regional or local scales, globally consistent assessments are required to support international disaster risk reduction efforts, and can serve as a reference for local and regional studies. This study presents a global-scale probabilistic tsunami hazard assessment (PTHA), extending previous global-scale assessments based largely on scenario analysis. Only earthquake sources are considered, as they represent about 80% of the recorded damaging tsunami events. Globally extensive estimates of tsunami run-up height are derived at various exceedance rates, and the associated uncertainties are quantified. Epistemic uncertainties in the exceedance rates of large earthquakes often lead to large uncertainties in tsunami run-up. Deviations between modelled tsunami run-up and event observations are quantified, and found to be larger than suggested in previous studies. Accounting for these deviations in PTHA is important, as it leads to a pronounced increase in predicted tsunami run-up for a given exceedance rate.Published219-2446T. Studi di pericolosità sismica e da maremot

Rupture Process of the 2004 Sumatra-Andaman Earthquake

The 26 December 2004 Sumatra-Andaman earthquake initiated slowly, with small slip and a slow rupture speed for the first 40 to 60 seconds. Then the rupture expanded at a speed of about 2.5 kilometers per second toward the north northwest, extending 1200 to 1300 kilometers along the Andaman trough. Peak displacements reached ~15 meters along a 600-kilometer segment of the plate boundary offshore of northwestern Sumatra and the southern Nicobar islands. Slip was less in the northern 400 to 500 kilometers of the aftershock zone, and at least some slip in that region may have occurred on a time scale beyond the seismic band